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A221651
Numbers divisible by their first digit squared (excluding those whose first digit is 1).
4
20, 24, 28, 36, 48, 50, 200, 204, 208, 212, 216, 220, 224, 228, 232, 236, 240, 244, 248, 252, 256, 260, 264, 268, 272, 276, 280, 284, 288, 292, 296, 306, 315, 324, 333, 342, 351, 360, 369, 378, 387, 396, 400, 416, 432, 448, 464, 480, 496, 500, 525, 550, 575
OFFSET
1,1
COMMENTS
Numbers where floor(n/10^floor(log(n)))^2 divides n.
LINKS
Christian N. K. Anderson, Table of n, a(n) for n = 1..10000
Christian N. K. Anderson, Ulam Spiral for the first 10000 terms. [Draw the usual Ulam spiral, containing all the nonnegative integers, and color the numbers belonging to this sequence yellow]
EXAMPLE
48 is divisible by 4^2.
PROG
(R)
x=0; y=rep(0, 1000); len=0
firstdig<-function(x) as.numeric(substr(as.character(x), 1, 1))
isint<-function(x) x==as.integer(x)
while(len<10000) if((fd=firstdig((x=x+1)))>1) if(isint(x/fd^2)) y[(len=len+1)]=x
KEYWORD
nonn,base
STATUS
approved